1. Field of the Invention
The present invention relates to a mobile communication terminal, such as a mobile phone and a personal digital assistant (PDA). More specifically, the present invention relates to a mobile communication terminal connectable to a plurality of base stations different in communication rate.
2. Description of the Related Art
Heretofore, the cdma2000 1x system by Qualcomm has been employed as a mobile communication scheme. This system widens the band in the cdmaOne system and realizes a maximum communication rate of 163.2 Kbps in a downlink direction (direction form base station to mobile communication terminal) and a maximum communication rate of 86.4 Kbps in an uplink direction (direction from mobile communication terminal to base station).
On the other hand, recently, the cdma2000 1x-EVDO system has been developed as a high-data-rate mobile communication scheme. The cdma2000 1x-EVDO system is a standardized high-data-rate (HDR) system which is an expanded system of cdma2000 1x. And the cdma2000 1x-EVDO system is standardized as STD.T-64 IS-2000 C.S.0024 “cdma2000 high-rate packet data air interface specification” of the Association of Radio Industries and Businesses (ARIB) in Japan. EVDO is an abbreviation of ‘evolution data only’.
The cdma2000 1x-EVDO system switches a modulation system of data to be transmitted from the base station to the mobile communication terminal on the basis of information showing a reception quality of the mobile communication terminal notified therefrom. Thereby, the cdma2000 1x-EVDO system can perform data communication by using a communication rate which is high in speed but low in error resistance when the reception quality in the mobile communication terminal is excellent, or by using a communication rate which is low in speed but high in error resistance when the reception quality in the mobile communication terminal is deteriorated. The communication rate in the cdma2000 1x-EVDO system becomes a maximum of 2.4 Mbps in the downlink direction and a maximum of 153.6 Kbps in the uplink direction and the cdma2000 1x-EVDO system can perform a significant high-speed data communication in comparison with the cdma2000 1x system.
By the way, in a current system, as disclosed in, for example, Japanese Patent Application Publication (KOKAI) No. 2002-345019, both base stations using the cdma2000 1x system and the cdma2000 1x-EVDO system are employed, therefore, even in the mobile communication terminal, a hybrid terminal equipped with a function corresponding to above-mentioned both systems has been developed. The use of this kind of communication terminal realizes a constantly stable data communication by selecting the aforementioned cdma2000 1x system and cdma2000 1x-EVDO system in response to communication rates which can be expected at each time.
However, the conventional mobile communication terminal has the following problems. That is to say, when the mobile communication terminal moves from a radio area where the communication terminal can use the cdma2000 1x-EVDO system to a radio area where the communication terminal cannot use the cdma2000 1x-EVDO system, so-called hand-off, in which a base station being a communication destination is switched from the base station in the cdma2000 1x-EVDO system to the base station in the cdma2000 1x, is performed. And in a state where the communication terminal performs a communication in the cdma2000 1x-EVDO system, even when the communication quality at the communication terminal deteriorates, the base station of the communication destination is switched to the base station in the cdma2000 1x system in any condition. Therefore, for example, if it is assumed that the communication terminal moves while downloading data from the base station in the cdma2000 1x-EVDO system and the base station of the communication destination is handed off from the base station in the cdma2000 1x-EVDO system to the base station in the cdma2000 1x. Then, a transmission rate of the data is extremely lowered and an unexpected time is taken for downloading the data sometimes, and this situation gives much annoyance to a user of the terminal.